This invention relates generally to the creation of an electric arc across an air gap and more specifically to a new and improved means to initiate an electric arc discharge which is sustained and augmented by a large power supply. The invention is disclosed in the context of an unbalance correction system.
In recent years advances in electronic vibration measurement equipment have made possible accurate determination of the amount and location of minute unbalance in rotating elements such as gyroscope rotors and small turbine wheels. Such rotating elements are often required to be balanced to levels approaching the limits of the measuring equipment. But while it is possible to electronically measure the quantity of material creating unbalance, no means has yet been devised for physically removing the exact minute amounts indicated. Consequently trial and error correction is employed. An estimated amount of stock is removed manually by milling, grinding, or by electrochemical methods. Unbalance is then measured to determine the effect of the correction and how much unbalance remains. This two-step process of alternate correction and measurement continues until an acceptable state of balance is attained.
Ideally, correction should occur simultaneously with measurement in a single operation. By continuously monitoring the effect of gradual stock removal lower levels of unbalance can be achieved in less time.
Rotating elements not requiring critical balancing would also benefit from such an ideal in-process balancing system in cases where controlled stock removal methods such as drilling or welding metal strip are not suitable. Even when controlled correction methods are utilized, in-process balancing may function for trim balancing to a precision not otherwise attainable except by the trial and error method previously described.
The present invention in one respect relates to a new and improved unbalance correction system wherein correction is attained by electric arc discharge through a heavy spot on a rotating mass to remove material from the heavy spot by vaporization.
The concept of starting an arc with a superimposed high voltage has long been utilized in automatic arc welding equipment. As applied in the balancing field the principle is found in prior art U.S. Pat. No. 2,322,561 which indicates use of high voltage to initiate a low voltage capacitor discharge across an air gap for stock removal purposes.
The invention herein claimed constitutes a new and improved means for triggering discharges across an air gap which finds particular utility for in-process balancing operations.
More specifically, it provides in-process correction of unbalance by means of periodic electrical discharges occurring between stationary electrodes and a workpiece rotating in a balancing machine of conventional design. Heat generated by the high-current arc so formed melts and vaporizes stock from the heavy spot of the workpiece. Each discharge results in an incremental reduction of unbalance and proceeds automatically to a minimal level of unbalance. The residual unbalance corresponds at most to the amount of material removed per discharge. Unbalance will never exceed this minimal level because the location of unbalance will change as soon as over-correction occurs. Thus it is impossible to over-shoot, and the system is inherently self-limiting once the minimal level is reached.
In the preferred embodiment, two electrodes are positioned in close proximity to the workpiece and to each other. The electrodes are designated positive and negative and are connected via a heavy cable to positive and negative terminals respectively of a capacitor bank without any intervening switching devices. An electrical circuit is completed from the positive to the negative electrode by imposition of the workpiece which functions as an intermediate passive conductor. This circuit comprises the workpiece and the air gap which exists on opposite sides of the workpiece, the electrodes being spaced from the workpiece.
Discharge of the capacitor bank occurs in response to a signal from the balancing machine's electronic vibration measurement instrument. This signal is synchronous with vibration from the unbalanced workpiece and is phase adjustable to time the discharge when the heavy spot is immediately adjacent the electrodes. A repeat cycle timer permits discharges to occur at spaced intervals rather than once per revolution to permit re-charging time of the capacitor bank.
Initial breakdown of the gap typically requires over 10,000 volts. The capacitors of the capacitor bank however are typically charged to only several hundred volts. Once breakdown of the gap has been initiated, only about 50 volts is required across the gap to sustain the high current flow. Breakdown is initiated by superimposing a high breakdown voltage upon the low-voltage/high-current main arc circuit to establish an ionized path across the gap. The invention provides a means for developing and applying such a high voltage to the gap to initiate breakdown and allow for the subsequent discharge of the capacitors to sustain and augment the electric arc. Once started the discharge is self-sustaining until the capacitor bank charge voltage is lowered to the gap maintaining potential of about 50 volts.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.